If you use eAccelerator and have updated your php5 version you need to recompile the plugin.
First step is to go in to the source listing of your eAccelerator and follow the instructions:
In my case the source is located in /usr/src/.
Don’t forget to restart your apache:
How to put in it in the first place? See article Install and Integrate eAccelerator into PHP5 on Debian Squeeze